334 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



[116]. It is fine-grained, yet shows distinctly to the naked eye all its prominent con- 

 stituents. Feldspar, a large part of which is clearly plagioclase, subordinate quartz, 

 hornblende in prisms with occasional terminations, a little brown biotite, yellow titan- 

 ite, and dark ore grains are all easily recognized. The microscope shows zircon and 

 apatite in addition, while chlorite and epidote are seen to result from the alteration of 

 both biotite and > hornblende. Muscovite forms in the orthoclase, which here seems 

 ,much more attacked than the plagioclase. There is no groundmass and of the 

 essential constituents only hornblende is developed in crystal form. 



A very similar dicrite was obtained from a prospect tunnel in French gulch, Lake 

 County [115], in which pyrite replaces magnetite as the ore and zircon and titanite are 

 very abundantly developed. Biotite and quartz are even less prominent than in the 

 preceding rock. 



AUGITE-BKAR1NG DIOEITE. 



In the Bed amphitheater, on the northeast side of Buckskin gulch, there occurs a 

 dike of a darker, finer-grained diorite than either of the preceding types [118]. Horn- 

 blende, biotite, plagioclase, and a little quartz may be macroscopically detected. The 

 microscope shows zircon, titanite, magnetite, hematite, apatite, biotite, augite, horn- 

 blende, plagioclase, orthoclase, and quartz. Augite appears most abundantly in the 

 freshest specimen, and certainly undergoes alteration to green hornblende, which, 

 though not fibrous, like typical uralite, is still by no means so compact as the common 

 dioritic hornblende. It is not possible, from the specimens examined, to say with cer- 

 tainty that any of the hornblende is original, although the association of the minerals 

 in the freshest specimens is such as to indicate a contemporaneous .formation of bio- 

 tite, augite, and hornblende. The latter two occur in irregular grains and the augite 

 has none of the pinkish tinge common to it when appearing in diorite. This rock is 

 remarkable as the only eruptive of the district in which augite has been found. 



Plagioclase appears abundantly in small grains, while orthoclase and quartz form 

 the cementing material. Chlorite and epidote result from the alteration of hornblende 

 and biotite; muscovite and calcite, from the feldspars. 



PORPHYRITE. 



Under this heading will be discussed a large number of distinct occurrences, 

 which, unlike those of the quartz-porphyries, belong for the most part to small rock 

 masses. There are in this group no markedly prevailing types to which the difl'ereut 

 rocks can be assigned, and the chief interest here lies in noting the great variations 

 possible, both in structure and composition, in what are practically equivalent masses. 

 One distinction, however, is feasible, viz, that between a variable subgroup, in which 

 a triclinic feldspar is evidently strongly predominant, and a few rocks occurring in 

 larger masses, in which orthoclase is also prominent and which seem at first glance 

 more nearly related to the quartz-porphyries than to the marked plagioclase rocks of 

 the first division. These latter types are referred to in the main report as quartz- 

 porphyries, and are so represented upon the map. They are called by the local names 

 Sacramento Porphyry and Silverheels Porphyry. Later investigation has shown 

 them to be plagioclastic rocks, and as such they will here be treated. In describing 

 them the general and variable group will first be considered and then the local types. 



